Thermal ablation or coagulation of the interior lining of a body organ is a procedure which involves heating the organ lining to a temperature that destroys the cells of the lining tissue. Such a procedure may be performed as a treatment to one of many conditions, such as menorrhagia, which is characterizes by chronic bleeding of the endometrial tissue layer of the uterus. Existing methods for effecting thermal ablation of the endometrial lining tissue include circulation of heated fluid inside the uterus (either directly or inside a balloon placed in the uterus), laser treatment of the lining, and resistive heating using application of RF energy to the tissue to be ablated. Techniques using RF energy provide an RF electrical signal to one or more electrodes in contact with the subject organ tissue. Electrical current flows from the electrodes and into the organ tissue. The current flow resistively heats the surrounding tissue. Eventually, the heating process destroys the cells surrounding the electrodes and thereby effectuates ablation.
For example, U.S. Pat. No. 5,769,880 (Truckai et al.) and U.S. Pat. No. 6,508,815 (Strul et al.) describe a system for endometrial lining tissue ablation using an electrode carrying member to transmit radiofrequency (RF) energy to cause thermal heating (and, thus ablation) of the tissue, wherein the electrode carrying member is substantially absorbent or permeable to moisture and gases such as steam, and is conformable to the uterine cavity. Suctioning means may additionally be positioned within the electrode carrying member to aid in the removal of moisture, whether gas or liquid, present or generated during the ablation procedure. An array of electrodes is mounted to the surface of the electrode carrying member, and arranged to produce ablation to a predetermined depth. The electrodes may be provided with means for variably controlling ablation depth by changing the electrode density or center to center spacing. Following placement of the ablation device within the patient's uterus, so that the electrodes are in contact with the tissue to be ablated, an RF generator is used to deliver RF energy to the electrodes and to thereby induce current flow from the electrodes to tissue to be ablated. As the current heats the tissue, moisture (such as steam or liquid) leaves the tissue causing the tissue to dehydrate. The moisture permeability or absorbency of the electrode carrying member allows for moisture to leave the ablation site so as to prevent the moisture from providing a path of conductivity for the current. The systems, devices and methods disclosed and described in U.S. Pat. Nos. 5,769,880 and 6,508,815 are well-suited for performing endometrial tissue ablation procedures, e.g., for treating Menorrhagia, the medical term for excessively heavy menstrual bleeding, and are embodied in the NovaSure® endometrial ablation system manufactured and distributed by Hologic, Inc., based in Bedford, Mass. U.S. Pat. Nos. 5,769,880 and 6,508,815 are both hereby fully incorporated by reference.
However, use of the NovaSure endometrial ablation system requires the operator to first measure the length and width of the uterine cavity, and input these dimensions into the controller in order to establish the power level for conducting the endometrial lining ablation procedure. This is because the amount of endometrial lining tissue to be ablated is proportional to the size of the uterine cavity. Additionally, procedure time can vary from as short as approximately one minute in duration, to as long as two minutes in duration, depending on a number of factors e.g., the amount of moisture in the tissue, in addition to the uterine cavity size.
In particular, any RF tissue ablation system must accurately determine the appropriate level of applied power for and during a procedure. This power level must provide sufficient heating to effectuate a complete, (i.e., uniform thickness) ablation. At the same time, however, the power level must be controlled to prevent over-ablation. Moreover, an RF generator must be controlled to respond dynamically to changes in the impedance of the subject tissue.
U.S. Pat. No. 5,954,717 (Behl, et al.) discloses and describes a system and methods for heating solid body tissue (e.g., a tumor) by delivering radio frequency energy through tissue electrodes, wherein the delivered energy is initially controlled to cause an abrupt increase in impedance between the electrodes and the tissue, observed in the form of an abrupt decrease in power delivered to the electrodes. The power at which the impedance increases or the time required to induce such an increase in impedance, are relied upon to determine a desired procedure power level for achieving a maximum sustainable delivery of radio frequency energy to the tissue to achieve complete and uniform heating of the solid tissue volume.
U.S. Pat. No. 6,033,399 (Gines) discloses and describes an electrosurgical generator having an output power control system that employs tissue impedance feedback to cause the impedance of vessel tissue to rise and fall in a cyclic pattern until the tissue is desiccated. The stated advantage of the disclosed power control system is that thermal spread (i.e., non-uniform heating) and tissue-charring (i.e., overheating) are reduced. The output power is applied cyclically by a control system with tissue impedance feedback. The impedance of the tissue follows the cyclic pattern of the output power several times, depending on the state of the tissue, until the tissue becomes fully desiccated. High power is applied to cause the tissue to reach a high impedance, and then the power is reduced to allow the impedance to fall, whereby the delivered thermal energy dissipates during the low power cycle. The control system is said to be adaptive to the tissue in the sense that output power is modulated in response to the impedance of the tissue.
U.S. Pat. No. 6,843,789 (Goble discloses and describes an apparatus for use in performing ablation of organs (such as the uterus), and other tissues, including a radio frequency generator which provides a radio frequency signal to ablation electrodes. The power level of the radio frequency signal is determined based on the subject area of ablation, and is coupled with the ablation electrodes through a transformation circuit. The transformation circuit includes a high impedance transformation circuit, and a low impedance transformation circuit. The high or low impedance transformation circuit is selected based on the impedance of the ablation electrodes in contact with the subject tissue. Vacuum level, impedance level, resistance level, and time are measured during ablation. If these parameters exceed determinable limits the ablation procedure is terminated.
U.S. Pat. No. 6,843,789 (Goble) discloses and describes an electrosurgical system, including an electrosurgical generator and a bipolar electrosurgical instrument, the generator being configured (i.e., programmed) to perform a tissue treatment cycle in which radio frequency energy is delivered to the electrosurgical instrument as an amplitude-modulated radio frequency power signal in the form of a succession of pulses characterized by successive pulses of progressively increasing pulse width and progressively decreasing pulse amplitude. There are periods of at least 100 milliseconds between successive pulses, and the treatment cycle begins with a predetermined pulse mark-to-space ratio. Energy delivery between pulses is substantially zero. Each burst is of sufficiently high power to form vapor bubbles within tissue being treated and the time between successive pulses is sufficiently long to permit condensation of the vapor.
U.S. Pat. No. 8,152,801 (Goldberg, et al.) discloses and describes solid tissue ablation systems and methods, in which radio frequency energy is delivered to the tissue, and a physiological parameter (e.g., impedance or temperature) indicative of a change in moisture concentration of the tissue is sensed. The ablation energy is alternately pulsed on and off to generate an energy pulse train, with the ablation energy being pulsed ON if the sensed physiological parameter crosses a threshold value indicative of an increase in the moisture concentration, and pulsed OFF if the sensed physiological parameter crosses a threshold value indicative of a decrease in the moisture concentration.
U.S. Pat. No. 8,241,275 (Hong et al.) discloses and describes a method of applying ablation energy for transmural tissue wall ablation, e.g., for treating afibrillation, including applying ablation energy at a starting power to a tissue site and monitoring the impedance of the tissue site. Thereafter, the power applied to the tissue site is reduced as a function of a rate of an increase in tissue impedance.
However, none of the foregoing references addresses the specific problems encountered in endometrial lining tissue ablation procedures, for example, accounting for differences in uterine size, or otherwise providing for a uniform and complete endometrial lining ablation in a substantially uniform time period, regardless of differences in uterine cavity size, tissue moisture, or other factors.